Although applications for the invention may be found in the automobile industry, specifically for electric and/or hybrid vehicles, the invention is by no means limited to these applications. The invention might be deployed, for example, in embedded systems or other systems.
A battery comprises a set of electrochemical storage cells, or electrochemical accumulators, such as lithium-ion cells, arranged in series and/or in parallel.
It is endeavored to maintain each cell within a restricted voltage range, in order to prevent irreversible deterioration. For example, in the case of a lithium-ion battery, there is a risk of fire where the cell voltage exceeds a maximum threshold, and a risk of irreversible deterioration in the cell where the voltage falls below a minimum threshold.
In practice, where a large number of cells are assembled in a pack, for example 96 cell pairs in battery packs for an electric vehicle, the cells are likely to show slightly different characteristics, some of which, such as the auto-discharge current, the Faradaic yield when charged, the maximum charge, etc. may have a direct influence upon the charging state of cells. Accordingly, where a battery pack is charged, variations in the level of charge may be observed from one cell to another.
An arrangement is known whereby each cell or tier of cells respectively is connected in parallel to a resistance via a switch, in order to permit discharging in case of the excessive charging of a cell. Dissipative balancing describes the dissipation of surplus energy as heat.
The management of energy transfers from the most highly-charged cells to other cells is also known. Non-dissipative balancing systems of this type may be advantageous, in that thermal radiation is lower, thereby preventing any potential damage to adjoining cells and electronic circuits. Moreover, these non-dissipative balancing operations, if executed during the discharging of the battery, may permit the optimum distribution of individual cell charges, such that the minimum charging state of each cell is achieved on a substantially simultaneous basis. This may extend the battery service life.
Battery charge balancing devices are known comprising converters, whereby each converter permits the recharging of one cell at a time. So as not to increase the number of converters, systems may be arranged for the selection of the cell to be charged or discharged, specifically based upon transistors, whereby the secondary side of the converter is connected to the selected cell. However, these selection systems may generate additional losses.
The simultaneous charging of multiple cells is also known. A battery charge balancing system is known, comprising a converter for the simultaneous recharging of multiple cells in a battery. For example, document FR2956260 describes a system of this type. In this document, the converter functions as a voltage generator, the level of which is regular, thereby delivering a predetermined balancing current to each cell. The main drawback of this system is the fact that the converter recharges the cells at a constant charging current, regardless of the number of cells to be recharged, thereby resulting in an equal number of converter service capacity levels to the number of cells which may be recharged simultaneously: a first level, corresponding to the recharging of 1 cell only, a second level corresponding to the recharging of 2 cells at a time, a third level corresponding to the recharging of 3 cells at a time, etc. This means that, in the majority of cases, the converter is operating far from its optimum service capacity, thereby resulting in substantial energy losses. A further drawback is the fact that operation at a constant current does not permit the more rapid recharging of a small number of cells which have been discharged to a particularly low level, with a potential restriction on the performance of the balancing system.
Accordingly, there is a need for a more efficient balancing system.